This invention relates to a method of applying a seal to catalyst monolith structures used in catalytic converters. More particularly it relates to an apparatus and a method for the extrusion of a seal coating onto the outer surface of such structures.
Catalytic converters are used for catalytically treating the exhaust gas of internal combustion engines. A conventional catalytic converter includes a relatively fragile ceramic monolithic catalyst structure which has a catalyst such as platinum deposited thereon. The catalyst structure typically has a cylindrical side surface and open functional end surfaces. Exhaust gases are treated by passing them through the catalyst structure from the input end to the output end. In a catalytic converter the catalyst structure is mounted within a metal housing.
A seal is disposed between the metal housing and the catalyst structure to support the monolithic catalyst structure within the housing. The seal fills the space between the housing and the catalyst structure so that gases can not leak through this space and so that substantially all of the exhaust gases entering the inlet end of the catalytic converter will pass through the catalyst structure. In addition to preventing gases from bypassing the catalyst structure, the seal holds the catalyst structure in place within the housing, thereby cushioning this relatively fragile ceramic structure to prevent breakage resulting from physical shocks due to mechanical vibration typical of automotive vehicles.
The seals used in catalytic converters are often made of a paste is coated onto the exterior lateral surface of the catalyst structure. This seal material must be able to withstand temperatures in excess of 2000xc2x0 F. since the catalytic converter will be exposed to such temperatures during operation. It must also be capable of holding the catalyst structure in place within the housing while dampening mechanical vibration between the housing and the catalyst structure.
Such a temperature resistant seal may, for instance, be made from a mixture in which an aqueous dispersion of high aspect ratio vermiculite is blended with ceramic fibers to produce a relatively smooth formable blend of vermiculite and fibers. A paste formed of this mixture is applied to the exterior lateral surface of the monolithic catalyst structure so that it encircles the structure while leaving the functional end surfaces uncovered so that exhaust gas can pass through.
Examples of mixtures suitable for such seals are disclosed in U.S. Pat. Nos. 5,207,989 and 5,385,873 and in British Patent No. 1,522,646, the disclosures of which are incorporated herein by reference. One example of such a material is a mixture comprising between approximately 28 and 80 dry weight percent of ceramic fibers capable of withstanding continuous exposure to temperatures in excess of 2,000xc2x0 F., between approximately 20 and 50 dry weight percent of an aqueous dispersion of high aspect ratio vermiculite and preferably also between approximately 5 and 60 percent dry weight percent of low temperature expanded vermiculite.
In the prior art, the seal material is applied to the catalyst structures by placing the catalyst structure in a mold and injecting the seal material between the mold and the catalyst structure to coat the structure. The coated catalyst structure is then heated to remove liquids from the seal material, effectively curing it in place around the monolithic catalyst structure. This process is time consuming since each catalyst structure must be separately coated with the seal material. In addition, there is a danger of inadvertently applying the seal material to the functional end surfaces of the catalyst structure. Alternatively, the catalyst structure could be mounted within the housing of the catalytic converter and the seal material could be injected into the space between the catalytic structure and the surrounding housing. Injecting the material directly into the space between the housing and the catalyst structure can result in damage to the relatively fragile ceramic structure and/or can fail to provide a uniform seal between the structure and the housing, since the seal cannot be inspected as it may be in the molding operation.
It is an object of the present invention to coat monolithic catalyst structures with seal material by a high speed extrusion coating operation in which a number of monoliths are passed through an extrusion chamber in which they are coated with the seal mixture in a continuous operation. This continuous process methodology shortens the time needed to apply the seal coating to each of the monoliths, thereby resulting in faster production and a cost savings.
It is a further object of this invention to apply a uniform and smooth seal coating to the lateral sides of monolithic catalyst structures, especially without having to coat each monolith separately in a mold.
Another object of this invention is to apply a seal coating to the lateral sides of the monolithic catalyst structures in a manner that will protect the functional end faces of the catalyst structures from being contaminated by the seal material.
Other objects, features and advantages of the invention will become apparent from the following description, when considered in connection with the accompanying drawing.